Technique to diagnose and control ammonia generation from a twc for passive ammonia scr operation

Abstract

A method for controlling ammonia generation in an exhaust gas feedstream output from an internal combustion engine equipped with an exhaust aftertreatment system having a first aftertreatment device includes executing an ammonia generation cycle to generate ammonia on the first aftertreatment device. The ammonia generation cycle includes monitoring an air-fuel ratio in the exhaust gas feedstream at a first location in the exhaust aftertreatment system, and monitoring an air-fuel ratio in the exhaust gas feedstream at a second location in the exhaust aftertreatment system. The air-fuel ratio at the first location is compared to the air-fuel ratio at the second location. If the air-fuel ratio at the second location is richer than the air-fuel ratio at the first location, operation of the engine is adjusted until the air-fuel ratio at the second location is equal to the air-fuel ratio at the first location.

1 . Method for controlling ammonia generation in an exhaust gas feedstream output from an internal combustion engine equipped with an exhaust aftertreatment system including a first aftertreatment device, comprising:
 executing an ammonia generation cycle to generate ammonia on the first aftertreatment device, said ammonia generation cycle comprising;
 monitoring an air-fuel ratio in the exhaust gas feedstream at a first location in the exhaust aftertreatment system; 
 monitoring an air-fuel ratio in the exhaust gas feedstream at a second location in the exhaust aftertreatment system; 
 comparing the air-fuel ratio at the first location to the air-fuel ratio at the second location; and 
 if the air-fuel ratio at the second location is richer than the air-fuel ratio at the first location, adjusting operation of the engine until the air-fuel ratio at the second location is equal to the air-fuel ratio at the first location. 
   
     
     
         2 . The method of  claim 1  wherein the ammonia generation cycle to generate ammonia on the first aftertreatment device comprises:
 operating the engine to generate an engine-out exhaust gas feedstream including nitric oxide, carbon monoxide, hydrogen and unburned hydrocarbons that converts to ammonia on the catalytic device. 
 
     
     
         3 . The method of  claim 2  wherein the engine is operated at one of a stoichiometric air-fuel ratio and a rich of stoichiometry air-fuel ratio. 
     
     
         4 . The method of  claim 1 , wherein the first location in the exhaust aftertreatment system comprises a location in the exhaust gas feedstream upstream of the first aftertreatment device, and wherein the second location in the exhaust aftertreatment system comprises a location in the exhaust gas feedstream downstream of the first aftertreatment device. 
     
     
         5 . The method of  claim 1  wherein the first aftertreatment device comprises a three-way catalytic device fluidly that is serially connected upstream of an ammonia-selective catalytic reduction device. 
     
     
         6 . The method of  claim 1  wherein the adjusting operation of the engine until the air-fuel ratio at the second location is equal to the air-fuel ratio at the first location comprises adjusting operation of the engine to generate an increased air-fuel ratio of engine-out exhaust gas feedstream until the air-fuel ratio at the second location is equal to the air-fuel ratio at the first location. 
     
     
         7 . The method of  claim 1  wherein the first aftertreatment device comprises two discrete elements positioned in series along a flow axis of the exhaust gas feedstream. 
     
     
         8 . The method of  claim 7  wherein the two discrete elements comprise a first discrete element including catalytic material comprising palladium and a second discrete element including catalytic material comprising palladium and rhodium and oxygen storage capacity material comprising at least one of cerium and zirconium. 
     
     
         9 . The method of  claim 1  wherein executing the ammonia generation cycle to generate ammonia on the first aftertreatment device is effected when an ammonia-selective catalytic reduction device fluidly serially connected downstream of the first aftertreatment device is ammonia depleted. 
     
     
         10 . The method of  claim 9  further comprising discontinuing the ammonia generation cycle to generate ammonia on the first aftertreatment device when the ammonia-selective catalytic reduction device has stored a predetermined amount of ammonia. 
     
     
         11 . Method for controlling ammonia generation in an exhaust gas feedstream output from an internal combustion engine equipped with an exhaust aftertreatment system including a three-way catalytic device and an ammonia-selective catalytic reduction device, comprising:
 executing an ammonia generation cycle to generate ammonia on the three-way catalytic device, said ammonia generation cycle comprising;
 operating the engine at a rich of stoichiometry air-fuel ratio to generate an engine-out exhaust gas feedstream including nitric oxide, carbon monoxide, hydrogen and unburned hydrocarbons that converts to ammonia on the three-way catalytic device; 
 monitoring an air-fuel ratio in the exhaust gas feedstream upstream of the three-way catalytic device; 
 monitoring an air-fuel ratio in the exhaust gas feedstream downstream of the three-way catalytic device; 
 comparing the air-fuel ratio upstream of the three-way catalytic device and the air-fuel ratio downstream of the three-way catalytic device; and 
 if the air-fuel ratio downstream of the three-way catalytic device is less than the air-fuel ratio upstream of the three-way catalytic device, adjusting operation of the engine to generate a leaner air-fuel ratio of engine-out exhaust gas feedstream until the air-fuel ratio downstream of the three-way catalytic device is equal to the air-fuel ratio upstream of the three-way catalytic device. 
   
     
     
         12 . The method of  claim 12  wherein the three-way catalytic device is fluidly serially connected upstream of the ammonia-selective catalytic reduction device, the three-way catalytic device further comprising:
 a front brick including catalytic material comprising palladium; and 
 a rear brick disposed downstream of the front brick including catalytic material comprising palladium and rhodium and oxygen storage capacity material comprising cerium and zirconium oxides. 
 
     
     
         13 . The method of  claim 11  wherein the ammonia generation cycle to generate ammonia on the three-way catalytic device is initiated when an ammonia generation condition is met. 
     
     
         14 . The method of  claim 13  wherein the ammonia generation condition is met when the ammonia-selective catalytic reduction device has not stored a predetermined amount of ammonia and predetermined opportunistic driving conditions are present. 
     
     
         15 . The method of  claim 11  wherein the adjusting operation of the engine to generate the leaner air-fuel ratio of engine-out exhaust gas feedstream until the air-fuel ratio downstream of the three-way catalytic device is equal to the air-fuel ratio upstream of the three-way catalytic device comprises the leaner air-fuel ratio of engine-out exhaust gas feedstream corresponding to one of a stoichiometric air-fuel ratio and a rich of stoichiometry air-fuel ratio. 
     
     
         16 . The method of  claim 11  further comprising:
 terminating the ammonia generation cycle to generate ammonia on the three-way catalytic device when an ammonia termination condition is met; and 
 in response to terminating the ammonia generation cycle, transitioning engine operation to operate at a non-ammonia generating condition. 
 
     
     
         17 . The method of  claim 16  wherein the ammonia termination condition is met when the ammonia-selective catalytic reduction device has stored a predetermined amount of ammonia and predetermined opportunistic driving conditions are not present. 
     
     
         18 . An exhaust aftertreatment system for an internal combustion engine, comprising:
 a catalytic device formulated to produce ammonia from an exhaust gas feedstream that includes nitric oxide, carbon monoxide, hydrogen and unburned hydrocarbons, the catalytic device close-coupled to an exhaust manifold of the internal combustion engine and fluidly coupled to an ammonia-selective catalytic reduction device located downstream of the catalytic device;   a first wide-range air-fuel ratio sensor located upstream of the catalytic device and a second wide-range air-fuel ratio sensor located downstream of the catalytic device, the first and second wide-range air-fuel ratio sensors each configured to generate a linear signal corresponding to air-fuel ratio over an air-fuel ratio range;   a control module configured to
 initiate an ammonia generation cycle to generate ammonia on the catalytic device; 
 monitor an air-fuel ratio in the exhaust gas feedstream upstream of the catalytic device; 
 monitor an air-fuel ratio in the exhaust gas feedstream downstream of the catalytic device; 
 compare the air-fuel ratio upstream of the catalytic device to the air-fuel ratio downstream of the catalytic device; and 
 if the air-fuel ratio downstream of the catalytic device is richer than the air-fuel ratio upstream of the catalytic device, adjust operation of the engine until the air-fuel ratio downstream of the catalytic device is equal to the air-fuel ratio upstream of the catalytic device. 
   
     
     
         19 . The exhaust aftertreatment system of  claim 18 , wherein the catalytic device includes a three-way catalytic device comprising:
 a front brick including catalytic material comprising palladium; and   a rear brick disposed downstream of the front brick including catalytic material comprising palladium and rhodium and including oxygen storage capacity material comprising cerium and zirconium oxides.